<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
    <select name="y" id="y">
        <script>
            for (var i = 1900; i < 2020; i++) {
                document.write("<option value="+i+">"+i+"</option>");
            }
        </script>
    </select>
    <select id="m" name="m">
        <script>
            for (let i = 1; i <= 12; i++) {
                    document.write("<option value='"+i+"'>"+i+"</option>")
            }
        </script>
    </select>
</body>
</html>